The invention relates to separation of two phases from each other, as well as an assembly suited for implementing said method. In particular, the invention concerns a method according to the preamble of claim 1 for separating solids and/or liquids from gas flows or, respectively, for separating solids from liquid flows. According to the present method, a gas-phase flow carrying, e.g., a catalyst or other solids or a liquid phase is passed to a separating means, wherein the other phase is then separated from said gas phase under the effect of a centrifugal force. To separate solids from a liquid-phase flow, the liquid flow is passed in a similar manner to a separating apparatus, wherein the solids are separated from the liquid under the effect of a centrifugal force.
The invention also relates to an apparatus according to the preamble of claim 15, suitable for separating solids and/or liquids from gas/liquid flows in fluidized-bed equipment.
Embodiments of highest commercial value used for separating two phases from each other are fluidized-bed reactors. Generally, fluidized-bed reactors are employed in the hydrocarbon conversion processes and energy generation. In these apparatuses a catalyst or similar solids-containing material capable of improving heat transfer or material fluidization is kept in a fluidized state by means of a gas-phase hydrocarbon or flue gas flow. Subsequently, the solids are separated from the gas flow by means of a cyclone.
The most generally used fluidized-bed reactor is a bubbling-bed reactor in which the linear flow velocity of the fluid medium is typically from 5 to 10 times the minimum fluidization velocity that can maintain the main portion of the solids in the fluidized bed of the reactor, whereby no significant amount of the solids can escape the reactor along with the hydrocarbon and flue gas flow. The term minimum bubbling velocity is defined as the linear gas flow velocity at which a portion of the gas flow begins to pass through the bed in the form of bubbles. This minimum bubbling velocity is dependent on the properties of the fluidizing gas and the solids involved.
When the gas flow velocity is increased above the minimum bubbling velocity, the top of the fluidized bed becomes less defined, in fact, being transformed into a gradient zone in which the solids content decreases upstream. At sufficiently high flow velocities, a fluidized flow is achieved in which practically all particulate solids are entrained in the gas flow that keeps up the fluidized state. Then, the solids separated by cyclones from the gas flow must be returned to the bottom part of the reaction space in order to maintain the mass balance unchanged.
As mentioned, the method and assembly according to the invention can be used, e.g., in processes employed for treating hydrocarbons. Examples of such processes include catalytic and thermal cracking, dehydrogenation, Fischer-Tropsch synthesis, manufacture of maleic acid anhydride and oxidizing dimerization of methane.
An application of the fluidized-bed reactor commonly used in energy generation is a boiler in which the fluidized material such as sand and/or solid fuel particles are fluidized with the combustion air flow and the flue gas released in the process. Also a liquid- or gas-phase fuel can be used. Circulating fluidized-bed (CFB) reactors of both the bubbling fluidized-bed and the entrained fluidization technique are conventionally used in the art. In these, the solids and unburnt particles are removed from the flue gas flow by means of cyclones. In this context, the term entrained fluidization refers to fluidization which takes place in both the turbulent fast fluidization range as well as the pneumatic transport range.
Hydrocarbon conversion processes are run using fixed-bed reactors and fluidized-bed reactors (fluidized catalytic reactors). In the present context, the term xe2x80x9cfluidized catalytic process equipmentxe2x80x9d is used to refer to equipment used in processes having a fine-grained pulverized catalyst suspended, e.g., in a slowly upward rising gas flow, wherein the catalyst promotes the occurrence of desired reactions.
One of the most widely employed fluidized-catalyst reactor systems in the art is the FCC equipment, that is, fluidized-catalyst cracking equipment, comprising chiefly a riser pipe acting as a reactor operated in the fast-fluidization flow state and a regenerator operated in the dense-phase bubbling bed state.
In fluidized-bed reactors, the particulate matter of the suspended solids and the product gas are separated from each other in cyclones utilizing the effect of the centrifugal force. Typically, a number of cyclones must be connected in series along the gas flow in order to improve the overall collection efficiency, because single cyclones of normal construction exhibit inferior separation capability for particles smaller than 15 xcexcm. Herein, a cyclone is rated effective if it can separate these small-diameter particles from the gas flow.
In addition to applications related to fluidized-bed reactors, cyclones are also used for, e.g., separating liquid droplets in steam systems, solids from flue gases of drying processes, phase separation on two-phase flows (demister equipment), separation of solids from gases (dust separators) and as hydrocyclones serving in the coarse separation of solids from waste waters.
Cyclone separators have either a coiled or spiralled structure in which the particulate matter suspension is directed as a tangential flow into the cylindrical section of the cyclone, whereby the catalyst particles are driven apart from the gas to a close distance of the cyclone inner wall when the flow typically circulates about 7-9 revolutions within the cylindrical section of the cyclone and the conical section forming a continuation thereof. Also axial cyclones are known in which the gas flowing through a pipe is forced into a circulating motion by means of vanes, whereby the solids under the centrifugal force are driven against the pipe wall and separated thereon from the gas flow.
The most common cyclone type is a single-port spiralled cyclone called the Zenz cyclone, in which the proportions of the different parts of the cyclone are standardized, thus permitting the dimensioning of the cyclone to be based on graphs and computational formulas. The collection efficiency of this cyclone can be enhanced by a large number of flow revolutions in the cyclone chamber, high flow rate at the inlet nozzle, higher density of solids, narrower inlet nozzle port and lower viscosity of the gas.
In the preseparation cyclone of a fluidized-catalyst cracking unit, tests have shown the gas residence time to be in the order of 1.0-2.0 s from the riser top to the cyclone outlet, after which the catalyst will further stay in the separation vessel at an elevated temperature for 5-40 s. During this time, valuable compounds will be lost as a consequence of thermal reactions. Resultingly, gasoline products will be converted by thermal cracking into combustible gases, particularly hydrocarbons of the C2 type. Other byproducts of thermal reactions are dienes, such as butadienes, which in the alkylation unit cause a significant increase in the acid consumption. Pentadienes in turn are particularly reactive, whereby their detrimental effect is evidenced as a reduced oxidation resistance of FCC gasoline. Further problems hampering the use of conventional FCC units are related to their poor control of reaction time and the erosion of the catalytic particles/circulating solids and the reactor structures.
The problems are mostly related to such essential parts of the equipment as the separation units of gases from solids/catalysts, that is, cyclones, which in most cases are implemented as single-port units. Herein, the term single-port cyclone refers to a cyclone construction having only one inlet nozzle for feeding the gas flow into the cyclone. To achieve the desired through-flow capacity, a plurality of these units are generally connected in parallel and then two or three in series.
In addition to being complicated and expensive, conventional cyclone constructions require a large footprint. Furthermore, the interior space of the cyclones must be lined with a ceramic compound to prevent erosion.
It is an object of the invention to overcome the disadvantages described above and to provide an entirely novel type of method and assembly for separating solids from a gas flow.
The goal of the present invention is attained by replacing at least one of the conventional cyclones of a fluidized catalytic process with a cyclone having multiple inlet openings (also known as a multi-inlet cyclone or a multiport cyclone), or alternatively, with a plurality of such multiport cyclones connected in series in a number of one or more. Herein, the term multiport cyclone is used to make reference to cyclone constructions having at least two, preferably at least 4 to 8 inlet ports for directing the gas flow to impact on the internal wall of the cyclone as an essentially tangential flow. The collection efficiency of a multiport cyclone can be made higher at low flow speeds and its structure is simpler and cheaper than that of conventional cyclones. Also the footprint required by the multiport cyclone is smaller.
A multi-inlet cyclone is mentioned the first time in a patent publication filed by E. I. Du Pont de Nemours and Company in 1974 (U.S. Pat. No. 3,969,096). Cited patent publication describes a cyclone separator having multiple-vaned gas inlet openings, said cyclone serving to separate suspended solid particles from internal combustion engine (in a car) exhaust gases.
However, E. I. Du Pont de Nemours and Company fails to present in the patent publication a theory capable of explaining why a multiport cyclone has a good collection efficiency at a low pressure drop. According to their hypothesis, the inlet guide vanes direct the incoming gas flow into the cyclone separator in sheetlike streams close to the inner wall of the cyclone shell, whereby the entrained particles need to travel a shorter distance prior to separation. Furthermore, the inventors assume that said sheets of inlet streams form a cleaner mass boundary between the downward and upward spiralling inlet streams, whereby the flow has a reduced tendency to form eddies. As stated in the application, reducing the eddy formation decreases the velocity-slowing drag on the inbound stream thus increasing the separating efficiency.
The separator equipment, or cyclones, used in the present invention comprise a cyclone chamber having an at least essentially upright aligned center axis and an advantageously essentially circular cross section of its internal space, whereby the separation chamber is rotationally symmetrical with respect to its center axis. To the separation chamber is connected an infeed nozzle of process gases, said nozzle having an essentially circular cross section centered about the center axis of the chamber. Further, the separation chamber includes a center pipe arranged therein for removal of gases and a downward return leg for the recovery of solids separated from the gas phase. The separation chamber is equipped with a set of guide vanes forming a louver which forces the gas to be treated into a stream circulating close to the inner wall of the cyclone chamber, thus effecting the separation of solids from the gas phase under the effect of the centrifugal force.
Advantageously, the assembly is comprised of cylindrical shells placed co-axially within each other, whereby the intershell channels with annular cross-section act as the fluidization space and the downward return leg of the reactor. The catalyst or solids are separated from the gas-phase suspension exiting the reactor by means of a multiport cyclone adapted immediately above the axially annular intershell flow channel.
Herein, the term xe2x80x9csolidsxe2x80x9d refers to the material forming the suspension in the reaction space. Typically, the solids consist of catalyst particles if the reactor is employed in catalytic reactions. When the reactor is used in physical or thermal processes, the solids may be either inert particulate matter serving to transfer beat or material into the reaction space or out therefrom, or alternatively, particles of a solid fuel. The catalyst is selected according to the process being run.
The multiport cyclone is advantageously connected to the upper part of the reaction space. The material to be treated in the cyclone is passed via multiple inlet openings into the cyclone chamber. The infeed openings can be located symmetrically or asymmetrically about the center axis of the cyclone. Advantageously, the openings are disposed symmetrically, and the riser space is given an annular cross section, whereby the flow is homogeneous over the entire cross section of the flow channel. In this case, the cyclone is equipped with flow guide vanes serving to the force the flow into the spiral motion required for the centrifugal separation. Typically, the guide vanes are adapted in a circularly louvered fashion about the perimeter of the cyclone chamber interior wall so as to form a louver comprising a plurality of parallel inlet channels for the entering gas flow. Thus, the infeed nozzle of a multiport cyclone comprises means for deflecting the infeed flow that enters the cyclone radially. Such means may be formed by, e.g., guide vanes adapted to the upper part of the cyclone so that at least some portion of the vane area deflecting the impinging flow causes the flow to assume an essentially high velocity component directed toward the cyclone center axis thus serving to direct the gas stream from the perimeter of the cyclone toward the center of the cyclone.
In a CYMIC circulating bed boiler developed by Kvaerner Pulping Oy (formerly Tampella Power Oy), such a multiport cyclone is used to remove entrained particles of the fluidized bed material from the flue gases and to return the particulate matter back to the boiler. The cyclone is disposed in the interior space of the boiler and is cooled with water.
It is possible to adapt a second multiport cyclone in the interior space of a first multiport cyclone or, alternatively, of a conventional cyclone, too, inasmuch the gas flow in the cyclone is symmetrical thus permitting the distribution of flow in a symmetrical manner to the guide vane system of the secondary cyclone. This kind of arrangement offers advantageous flow and construction properties, because the lower concentration of catalyst in the secondary cyclone permits the latter cyclone to be operated at a higher flow velocity than the preceding upstream cyclone. Depending on the available factory space and collection efficiency, a desired number of cyclones can be connected in series.
In a preferred embodiment of the invention, the infeed nozzle of essentially annular cross section used according to the invention for distributing the gases to be treated is implemented so that the means for deflecting the radially entering gas flow are extended in the radial direction outside the exterior perimeter of the cyclone. Furthermore, in a particularly advantageous embodiment of the invention, said means, such as the infeed nozzle comprising the guide vane system extends in the exterior space of the cyclone from the top level of the cyclone downward along the outer perimeter of the cyclone shell. Then, the portion of the guide vane system located on the outside surface of the cyclone and directed downward on the same can be adapted to direct the flow entering the cyclone in the upward direction from the preceding cyclone that surrounds the said cyclone. Flow direction in the present context is used for making reference to, e.g., flow guidance, stabilization and/or deflection. The guide vane system may also be placed only partially inside the inlet channel or, alternatively, entirely or only partially inside the cyclone.
In a preferred embodiment of the invention, the downward return legs of the concentrically adapted cyclones are placed in a similar manner co-axially. In a further preferred embodiment of the invention having at least two multiport cyclones adapted concentrically, the cyclones are advantageously designed so that the guide vane system of any inner cyclone is always located above the guide vane system of the respectively upstream preceding outer cyclone.
Accordingly, the goal of the invention is attained by adapting at least one multiport secondary cyclone inside a primary cyclone or another preceding secondary cyclone.
More specifically, the method according to the invention is characterized by what is stated in the characterizing part of claim 1. Furthermore, the assembly according to the invention is characterized by what is stated in the characterizing part of claim 15.
The present invention provides significant benefits. Accordingly, the equipment construction according to the invention, which is based on the use of a multiport cyclone, gives significant advantages in flow dynamics and process engineering over conventional arrangements and generally used single-port cyclones. This is because of the fact that in conventional single-port cyclones, the solids flow impinges on the cyclone inner wall as a homogeneous gas-suspended jet of high flow velocity which in primary cyclones is typically in the range 20-25 m/s, in secondary cyclones about 35 m/s, and in tertiary cyclones about 40 m/s. The flow rate of the impinging jet must be high, because the cyclone inlet nozzle width (et width) is generally, e.g., in standardized Zenz cyclones about one-fourth of the cyclone diameter, and the particulate matter must be brought over the entire width of the impinging jet close to the cyclone inner wall in order to achieve separation of the solids from the gas flow. In this type of cyclone, the point most susceptible to erosion is the area of the cyclone inner wall receiving the jet impact of the suspended catalyst particles.
By contrast, in the construction according to the invention, the erosion problems are eliminated by improved flow dynamics: the conventional single large-volume inlet flow of solids is divided into a plurality of smaller-volume mass flows impinging on the internal wall of the multiport cyclone, whereby the erosive effect is distributed over a larger area. By virtue of the multiport construction, the cyclone inlet ports can be made narrow, whereby the catalyst layer becomes shallow, and the flow velocity at any inlet port may be essentially smaller than in conventional single-port cyclones in which reduction of the inlet port width would require an increased channel height, resulting in a higher cyclone and requiring an infeed channel of an elongated and clumsy shape. The possibility of using a reduced cyclone inlet flow velocity contributes to a further lowered erosion rate, which according to published references is dependent on the flow velocity by a power of 4 to 5.
In tests carried out at room temperature, a cyclone according to the invention with 465 mm diameter with full-area inlet ports and straight vanes has demonstrated a collection efficiency of 99.99% at 5.6 m/s inlet flow velocity when the cross-sectional mass flow rate of the catalyst according to differential pressure measurements was over 200 kg/m2s. In a conventional Zenz cyclone with compatible dimensions and flow rates, the collection efficiency was 99.10% as computed by particle size fractions. A comparison of these collection efficiencies makes it clear that the novel cyclone with multiple narrow inlet ports according to the invention offers a superior efficiency when the design goal is to avoid high flow velocities leading to erosion.
In a preferred construction according to the invention having the reactor riser pipe (hereafter, shortly a riser) connected directly to the cyclone inlet pipe, an accurately controllable residence time is achieved, because the catalyst is made to enter the cyclone from each point of its infeed pipe simultaneously. Hence, a cyclone according to the invention can be designed for a volume about half of that of a standard cyclone. By placing the cyclones concentrically inside one another, the valuable interior volume of the cyclone pressure vessel can be reduced as compared to arrangements having the cyclones placed in a parallel or superimposed manner in the interior space of the pressure vessel. Since a cyclone according to the invention may have a shorter construction owing to its improved flow dynamics, its height and, respectively, retention time can be, e.g., halved from the corresponding values of a standard cyclone. Resultingly, the possibility of undesired thermal reactions is reduced. Moreover, the product can be cooled directly in the discharge pipe of the cyclone if so required.
According to a first preferred embodiment of the invention, the multiport cyclone is used for separating catalyst from the product gases of a fluidized catalytic cracking (FCC) process. The multiport cyclone may also be employed in the regenerator equipment of an FCC unit for separating the regenerated catalyst from the coke combustion gases.
Other suitable fluidized catalytic processes are, among others: catalytic reforming, oxidizing dimerization of phthalic acid anhydride, maleic acid anhydride or methane, Fischer-Tropsch synthesis, chlorination and bromination of methane, ethane and other hydrocarbons, and conversion of methanol into olefines or gasoline.
Separation of solids is carried out using a plurality (e.g., 2-10, most appropriately 2-5) of cyclones connected in series. By virtue of their structure, the cyclones used in the invention, of which at least one is a multiport cyclone, can be adapted concentrically inside one another, e.g., so that the downward return leg of the any one cyclone in the downstream series of cyclones is adapted to the interior of the downward return leg of the preceding cyclone. Owing to the longitudinally-stacked coaxial placement of the cyclones within the pressure shell, a significant volume reduction is attained with respect to conventional cyclone constructions requiring side-by-side placement of the cyclones. A multiport cyclone can be made with a larger diameter than a conventional cyclone; the diameter of multiport cyclones may be over one meter, even up to several meters, whereas the diameter of a conventional cyclone is generally limited to 1 m maximum. Yet, the diameter of the reaction vessel need not be increased in the embodiment according to the invention, but instead, may even be made smaller.
The cyclone infeed nozzle can be formed from the intershell space remaining between two concentrically adapted cylindrical or partially conical envelope surfaces, whereby said annular space may be divided into parallel flow segments by means of axially extending baffles. The parallel flow segments can be implemented by mounting longitudinally aligned baffles radially between the two coaxial, cylindrical envelope surfaces. Almost an equivalent result is obtained by constructing the infeed nozzle with the annular cross section from a set of parallel infeed channel tubes mounted equidistantly spaced in a circular fashion.
The guide vanes of the cyclone are adapted in a circularly louvered fashion about the perimeter of the cyclone chamber wall, partially or entirely inside the riser channel so as to form a louver comprising a plurality of parallel inlet channels for the entering gas flow.
The cyclone(s) according to the invention either is/are connected directly to the riser channel (shortly, riser) of a fluidized catalytic process reactor, which is a preferred embodiment of the invention, or alternatively, the infeed nozzle(s) of the cyclone(s) is/are adapted to communicate with the gas space of a fluidized catalytic process reactor as is the case with conventional arrangements.
In a preferred embodiment of the invention having the means, which are provided for deflecting the flow entering the cyclone in a radial direction, arranged to extend radially outward to the outer space of the cyclone, the flow can be controlled effectively already prior to its entry into the cyclone. Moreover, in a particularly advantageous embodiment of the invention, in which said means extend downward from the top level of the cyclone, the flow control effect may be further augmented and the flow control started earlier than in conventional constructions. Resultingly, it is possible to control efficiently and at an early stage the flow which leaves the vortex zone of the preceding outer cyclone and is directed upwards. Owing to the efficient flow control arrangement, the flow can be passed in a desired state of flow into the inner cyclone, unaffected by any possible irregularities in the flow pattern of the outer cyclone. Furthermore, the powerful flow control effect exerted by the guide vane system, particularly in its upright portion outside the inner cyclone, facilitates an exceptionally good degree of preseparation between the outer and inner cyclones of the separator assembly.
As the inlet flow to the inner cyclone may already initially have a tangential velocity component, it may be advantageous not to extend the guide vanes up to the outer edge of the inlet channel of the inner cyclone.
A further benefit is gained therein that the upright deflecting or guiding means adapted on the outer perimeter of the cyclone, said means comprising a tubular outer envelope for forming a gas flow channel in the intershell space between said outer envelope and said outer perimeter of said inner cyclone, facilitates an advantageous concentric placement of the multiport cyclones inside one another so that the guide vane system of each successively inner cyclone is located above guide vane system of the respectively preceding outer cyclone. Herein, it is easy to realize a construction in which the downward return leg of the inner cyclone contains a column of solids column extending higher than the solids column formed in a similar manner from the separated product solids in the downward return leg of the outer cyclone. The solids column must be maintained if the pressure in the interior space of the cyclone is lower than the ambient pressure about the bottom end of the cyclone downward return leg. Respectively, the height difference between the tops of the solids columns is necessary in order to compensate for the difference between the pressure levels in the interior spaces of the cyclones when the bottom ends of the cyclone downward return legs exit in the same space. The pressure difference between the interior spaces of the cyclones is principally created by the pressure drops occurring in the guide vane systems or similar deflecting means as well as by the pressure losses occurring in the flow channels and due to the changes of the flow velocity. The pressure difference is compensated for through the different hydrostatic pressures over solids columns of different heights accumulated in the downward return legs of the cyclones. In this manner, the return of the solids to the cyclone bed can be implemented using the embodiment described above.